A window formed in a security document is particularly suited for incorporating a security device, such as a diffraction grating or hologram. The usual method of incorporating a diffraction grating or hologram in a security documents is to emboss the required diffractive relief structure onto a transfer foil of metallic material, and to transfer the foil bearing the diffractive structure on to the required area of the security documents in a hot stamping operation. This is a relatively expensive process which is not particularly suitable for the mass production of security documents such as bank notes or the like.
Another method of producing diffractive structures in security documents is disclosed in our International Patent Application No. WO01/00418 in which an optically diffractive structure is formed by irradiation of an area of the substrate by laser ablation. Whilst such a laser ablation process can reduce the cost of providing diffractive structures in bank notes or the like, the process still involves the use of relatively expensive laser equipment.
It is therefore desirable to provide a security documents with a security device including an embossed relief structure, which can be integrated in to the document in a simpler, less expensive process.
It is also desirable to provide a security document or device of a more complex nature, which can produce different visual effects and/or be used for different purposes, such as for verifying a security feature at another part of the security document.
Further, many security documents, such as banknotes, have several different security features in different areas of the note. This can be confusing for the public, but is a necessity imposed by applying security features by different methods which requires relatively large tolerances, typically of at least 1.5-2.0 mm so that the features stand by themselves. It is therefore desirable to provide a process for forming multiple security features in a single area that does not require large tolerances and which is difficult to replicate.